Another tangential question I've been struggling with is: is there a human limit to a power density we'll tolerate? Any sufficiently powerful battery is a defect or bad actor away from becoming a weapon. We already see defects in lithium batteries shaping airline policy, and the devices we're talking about are peanuts if we want to enable all our sci-fi dreams.
It depends on how easy it is to release the energy. A full petrol tank already contains a scary amount of energy, but you can't release it instantanously since it is bottlenecked by oxygen intake for combustion. Explosives bring their own compact oxidizer to get around that problem.
A chunk of uranium-238 contains a lot of energy but it is only fissionable but not fissile, so you need an external fast neutron source to extract that energy. Comparatively it is a fairly safe energy source for its density. Similarly there are some metastable nuclear isomers that can theoretically be stimulated to release their energy with high energy photons with a precise frequency and would otherwise release their energy relatively slowly through their natural decay modes.
And I think the difficulty of turning a hypothetical portable fusion power plant into an explosive device should be evident from the difficulties of keeping current experiments running for even a few minutes.
I just though of an interesting twist on subcritical reactor. What about smelting a californium neutron source together with the fuel? We get a much cheaper RTG alternative with near no need for regulation.
There is a power limit already in place for batteries you can carry on airlines and IIRC it's 100 Wh.
As for weaponizing batteries, pretty sure bad actors have been able to cook dangerous bombs at home for decades that can inflict magnitudes more damage than a battery could ever do. So battery based bombs won't be a thing for the near future.
That is because of the current lithium chemistry batteries. A more stable battery that wont blow up so easily after some damage wont have the same restriction
